In 1981 co-inventor Faroudja patented a low level/signal booster for television transitions as represented by U.S. Pat. No. 4,262,304, reference to which is made for further particulars. The system and method set forth in the referenced '304 patent has worked very satisfactorily over the years in providing low level video transitions with shortened durations. However, as effective bandwidths for composite color video signals have continued to increase, within the standard formats such as NTSC and PAL, certain compromises and drawbacks present in the prior approach taught by the referenced '304 patent have come to light.
In the referenced '304 patent, because the shaper circuit 16 had a low pass filter characteristic, a compromise was required between purity of large signal response and sharpness of small signal response. With substantial low pass filtering at the shaper element 16 of the referenced '304 patent, the correction waveform F' is as shown in the left column of waveforms (high levels) of FIG. 2 thereof. However, the low level transition waveform G" of the right column was not as sharp as it would have been, because of the presence of the substantial low pass filter characteristic of the shaper circuit 16.
However, with a mild effect low pass filter characteristic at the shaper 16, the low level signal transitions exemplified by FIG. 2 waveform G" were considerably sharpened, with the concomitant considerable drawback that at high transition levels the high level correction signal F' was not flat as shown in FIG. 2, left column. Rather, the F' signal included sinusoidal amplitude pollution characteristics during the waveform C' excursions, leading to pollution of high level transitions in the video path.
Thus, the approach described in the referenced '304 patent required a compromise between sharpness of low transition level picture detail and ringing of high transition level picture detail; a compromise caused by the particular characteristics of the low pass filter selected for the shaper circuit 16. The results achieved by the methods and apparatus described in the referenced '304 patent were excellent for limited bandwidth applications; but are now becoming inefficient for many present day applications requiring a full bandwidth correction path under all transition level conditions.
The advantages of chrominance/luminance spectrum separation preprocessing and postprocessing techniques within a television system are discussed in the co-inventors, U.S. Pat. No. 4,731,660, reference to which is made for further particulars. It has been discovered by the present co-inventors that the referenced chrominance/luminance spectrum separation preprocessing and postprocessing techniques described in the referenced '660 patent are advantageously enhanced further by addition of the television signal transition booster of the present invention.